Document QSurfaceFormat::ColorSpace deprecation

Pick-to: 6.9 6.8 6.5
Change-Id: I894316d0b3df703e5fd07a1c18f579992215e621
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/kernel/qsurfaceformat.cpp

@@ -182,6 +182,7 @@ public:
 
 /*!
     \enum QSurfaceFormat::ColorSpace
+    \deprecated [6.0] Use setColorSpace(QColorSpace) instead
 
     This enum is used to specify the preferred color space, controlling if the
     window's associated default framebuffer is able to do updates and blending